1. Field of the Invention
The present invention relates to a semiconductor active fuse, and particularly, to a semiconductor active fuse appropriate for a high-voltage power supply controller.
2. Description of the Related Art
FIG. 1 shows an example of a power supply controller according to a related art. This power supply controller employs a transistor QF having a temperature sensor, for selectively controlling the supply of power from a power supply 101, such as a battery of a vehicle, to a load 102. In the example of FIG. 1, the power supply 101 is of a 12-V system and supplies a voltage VBp. The power supply 101 is connected to an end of a shunt resistor RS. The other end of the shunt resistor RS is connected to a drain terminal D of the transistor QF. A source terminal S of the transistor QF is connected to the load 102, which may be a headlight or a power-window driving motor. The power supply controller further has a control circuit 901, an A/D converter 902, and a microcomputer (CPU) 903. The control circuit 901 detects a current passing through the shunt resistor RS and controls the transistor QF through hardware circuits. The A/D converter 902 and microcomputer 903 turn on and off a drive signal for the transistor QF according to the current monitored by the control circuit 901. The transistor QF has a thermal protection function for forcibly turning off the transistor QF through an incorporated gate turn-off circuit. When detecting that the temperature of the transistor QF is above a specified temperature, the temperature sensor in the transistor QF informs the gate turn-off circuit of the high temperature, and the gate turn-off circuit forcibly turns off the transistor QF.
A Zener diode ZD1 keeps a voltage of 12 V between the gate terminal G and source terminal S of the transistor QF, and protecting an overvoltage breakdown, bypassing between the true gate TG and the source terminal of the transistor QF. The control circuit 901 includes differential amplifiers 911 and 913 serving as a current monitor circuit, a differential amplifier 912 serving as a current limiter, a charge pump 915, and a driver 914. The driver 914 receives an ON/OFF control signal from the microcomputer 903 and an overcurrent signal from the current limiter 912 and drives the gate G of the transistor QF through an internal resistor RG (not shown) accordingly. The differential amplifier 912 uses a voltage drop occurring at the shunt resistor RS to detect a current flowing to the transistor QF. If the detected current is an overcurrent above an upper threshold, the differential amplifier 912 instructs the driver 914 to turn off the transistor QF. Once the detected current becomes below a lower threshold, the differential amplifier 912 instructs the driver 914 to turn on the transistor QF. The microcomputer 903 always monitors a current through the current monitor circuit made of the differential amplifiers 911 and 913. Upon detecting an abnormal current exceeding a normal level, the microcomputer 903 issues an OFF signal to the transistor QF to turn off the transistor QF. If the temperature of the transistor QF exceeds a predetermined level before the microcomputer 903 issues the OFF signal, the thermal protection function turns off the transistor QF.
To detect a current, the related art must have the shunt resistor RS in a power supply line. If a current flowing through the shunt resistor RS is large, the shunt resistor RS will cause a large heat loss that is not ignorable.
The thermal protection function and overcurrent control circuit of the related art may work on a dead short that occurs in the load 102 or wiring to produce a large current. However, the related art unsatisfactorily works on an incomplete short circuit failure such as a layer short having a certain extent of short-circuit resistance to produce only a weak short-circuit current. Only way for the related art to cope with such an incomplete short circuit failure is to detect an abnormal current caused by the short circuit failure with the use of the microcomputer 903 and current monitor circuit and turn off the transistor QF by the microcomputer 903. The microcomputer 903, however, is slow to respond to such an abnormal current.
The shunt resistor RS, AID converter 902, and microcomputer 903 that are imperative for the related art need a large space and are expensive, to increase the size and cost of the power supply controller. When applied for a high-voltage power line, the microcomputer 903 must be protected from the high voltage, to further increase the size and cost of the power supply controller.
At present, power supply systems for vehicles are mainly of 12 volts. For the 12-V power supply system, it is sufficient to consider a maximum supply voltage of about 18 V. To reduce a power loss due to a load current, it is studied to increase the power supply system to 42 volts. To meet the 42-V supply voltage, the transistor QF and control circuit 901 must have a higher breakdown voltage.
If the supply voltage is increased to 42 V, the differential amplifiers 911 to 913 and driver 914 of the related art of FIG. 1 must also have an increased breakdown voltage. The elements 911 to 914 are manufactured through CMOS processes or BiCMOS processes, and these processes must be modified to increase the breakdown voltage of the elements. Increasing the breakdown voltage of a given device is achievable by increasing the number of elements having the same breakdown voltage, thickening a gate insulating film, or forming a guard ring or a field plate for improving the breakdown voltage of each element. The technique of increasing the number of elements having the same breakdown voltage increases a chip area and complicates manufacturing processes to increase costs. The technique of thickening a gate insulating film deteriorates the electric characteristics such as transconductance gm of each semiconductor element. In addition, operating the semiconductor elements under a high voltage deteriorates the reliability of the semiconductor elements.
It is advantageous, in terms of costs and reliability, if the control circuit 901 can still employ 12-V elements even if a higher supply voltage is introduced.
An object of the present invention is to solve the problems of the related art mentioned above and provide a semiconductor active fuse capable of quickly coping with an abnormal current caused by an incomplete short circuit failure such as a layer short without a shunt resistor.
Another object of the present invention is to provide a semiconductor active fuse capable of employing conventional 12-V elements for a comparator for comparing the potentials of the second main electrodes of first and second semiconductor elements with each other even if a 12-V power supply system is increased to, for example, a 42-V power supply system.
Still another object of the present invention is to provide a semiconductor active fuse capable of operating with a control circuit that employs a comparator of a conventional breakdown voltage, thereby avoiding a cost increase to be involved in increasing the breakdown voltage of the comparator.
Still another object of the present invention is to provide a semiconductor active fuse of improved reliability realized by a comparator of a control circuit that operates under a standard voltage, not requiring the bias condition for higher supply voltage.
In order to accomplish the objects, the present invention provides a semiconductor active fuse having a first semiconductor element that has a first main electrode connected to a DC power supply, a second main electrode connected to a load, and a control electrode, a second semiconductor element that has a first main electrode connected to the first main electrode of the first semiconductor element, a second main electrode connected to a reference circuit, and a control electrode connected to the control electrode of the first semiconductor element, a comparator that has a high-potential power supply terminal, a low-potential power supply terminal, a first input terminal connected to the second main electrode of the first semiconductor element, and a second input terminal connected to the second main electrode of the second semiconductor element, a driver for supplying a control voltage to the control electrodes of the first and second semiconductor elements according to the output of the comparator, a first diode connected between the first input terminal and the low-potential power supply terminal, and a second diode connected between the second input terminal and the low-potential power supply terminal through a resistor. If an overcurrent that is above a reference current determined by the reference circuit flows through the first semiconductor element, the active fuse turns on and off the first semiconductor element to produce current oscillations that cut off the conductive state of the first semiconductor element. The reference circuit may be a reference resistor, a constant current source, a parallel circuit of a resistor and a constant current source, or any other else. The high-potential power supply terminal is kept at a system supply voltage, and a voltage between the high-potential power supply terminal and the low-potential power supply terminal is kept at a voltage that is lower than the system supply voltage. The first and second semiconductor elements may be MOS transistors such as MOS field effect transistors (FETs) or MOS static induction transistors (SITs), or insulated gate power semiconductor elements such as insulated gate bipolar transistors (IGBTs). Alternatively, the first and second semiconductor elements may be MOS composite semiconductor elements such as emitter switched thyristors (ESTs). The first and second semiconductor elements are any one of n- and p-channel types. The first main electrode is any one of the emitter and collector electrodes of the IGBT, or any one of the source and drain electrodes of the MOS transistor. The second main electrode is the other of the emitter and collector electrodes, or the other of the source and drain electrodes. More precisely, if the first main electrode is the emitter electrode, the second main electrode is the collector electrode. If the first main electrode is the source electrode, the second main electrode is the drain electrode. The control electrode is the gate electrode of the IGBT or the MOS transistor.
Even if the 12-V power supply system is improved to, for example, the 42-V power supply system, the semiconductor active fuse of the present invention is capable of using 12-V elements for a comparator for comparing the potentials of the second main electrodes of the first and second semiconductor elements with each other. Namely, the present invention needs no increase in the breakdown voltage of the comparator, and therefore, no increase in the cost thereof.
In addition, the semiconductor active fuse of the present invention improves the reliability thereof by avoiding the use of the comparator under the higher voltage conditions.
Other and further objects and features of the present invention will become obvious upon an understanding of the illustrative embodiments about to be described in connection with the accompanying drawings or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employing of the invention in practice.